Field of the Invention
The present invention relates to a motor driven power steering system. More particularly, it relates to a control apparatus and a method of a motor driven power steering system which can enhance a steering sensation of a driver not only in a steering section in which the driver rotates a steering wheel from an on-center position to the left or the right so as to steer the vehicle, but also in a restoration section in which the rotated steering wheel rotates and returns to the on-center position.
Description of Related art
Generally, examples of a power assistance steering system for reducing steering force of a driver when steering a vehicle include a Hydraulic Power Steering (HPS) system which uses hydraulic pressure formed by a hydraulic pump to assist with the steering force of the driver, and a Motor Driven Power Steering System (MDPS) which uses output torque of an electric motor to assist with the steering force of the driver, and so forth.
In the MDPS system, the output of the electric motor for steering assistance in performing a steering assistance function depending on a steering wheel manipulation of the driver can be controlled depending on driving conditions of the vehicle. Therefore, the MDPS system provides further enhanced steering performance and steering sensation compared to those of the HPS system.
Given this, recently produced vehicles are widely provided with MDPS systems which can change and control assistance steering force generated by the output of the motor depending on driving conditions.
Such an MDPS system includes sensors such as a steering angle sensor which detects a steering angle (a column input angle) varying depending on steering wheel manipulation of a driver, a torque sensor which detects steering torque (steering wheel torque, column torque) inputted through the steering wheel, a vehicle speed sensor which detects the speed of the vehicle, a wheel speed sensor, an engine rpm sensor, a yaw rate sensor, etc., a controller (an MDPS ECU), and a steering motor (an MDPS motor).
In the above-mentioned configuration, the controller receives and obtains, to control the operation and output of the steering motor, driver steering input information such as a steering angle, a steering angular speed, steering torque, etc. and vehicle state information such as a vehicle speed, a wheel speed, an engine RPM, a yaw rate, etc.
The steering angle refers to the position of the steering wheel. The steering angular speed may be an angular speed which is obtained through a separate sensor or from a differentiated signal of a steering angle signal. The steering torque is torque which is applied from the driver to the steering wheel, in other words, is a driver input torque.
The controller controls the driving force of the steering motor depending on the vehicle speed to generate controlled assistance torque (hereinafter, referred to as ‘auxiliary steering torque’). In this regard, under conditions of a low vehicle speed, the motor output for steering assistance is increased to enable the driver to manipulate the steering wheel with comparatively small force. Under conditions of a high vehicle speed, the motor output is reduced to make the driver manipulate the steering wheel with comparatively large force so that the driving stability of the vehicle can be secured.
When the vehicle moves at a high speed, if the steering wheel is excessively light, a dangerous situation may be caused despite the fact that the driver manipulates the steering wheel with small force. Thus, the driving stability is reduced. Given this, steering assistance characteristics are changed depending on the vehicle speed, wherein when the vehicle moves at a high speed, auxiliary steering torque is applied such that the force required to control the steering wheel is increased, whereby the stability in controlling the steering wheel can be enhanced.
Typically, the output of the steering motor for assisting with the steering force of the driver is generated while the controller controls motor current (assist control current) to be applied to the steering motor.
In this regard, the controller is configured to determine, as it is tuned, current corresponding to an output value determined based on information basically collected from the vehicle, that is, driver steering input information and vehicle state information, and applies the determined current to the steering motor. Thereby, the steering motor is operated to generate auxiliary steering torque that is force for assisting with the steering force of the driver.
In the steering system, elements for transmitting the driver steering force to be applied through the steering wheel and the assistance steering force generated by the motor depending on the driver steering wheel, include a steering column which is installed under the steering wheel, a gear box which converts the rotating force transmitted from the steering column into linear force, and a universal joint which transmits the rotating force from the steering column to the gear box.
The gear box includes a pinion gear which receives the rotating force from the universal joint, and a rack bar on which a rack engaging with the pinion gear is provided. When the pinion gear rotates, the rack bar is linearly moved by the rack to the left or the right.
In this regard, force generated by the left or right linear movement of the rack bar is transmitted to tires through a tie rod and a ball joint, thus changing the direction of the tires.
As described above, in the MDPS system, a motor control value is determined using information collected from the vehicle, and the operation of the motor is controlled using the motor control value, whereby steering assistance is embodied.
In this regard, the auxiliary steering torque outputted from the motor in the MDPS system influences a steering sensation that the driver feels when manipulating the steering wheel.
FIG. 1 illustrates a hysteresis loop showing steering torque that the driver feels depending on the steering angle under certain vehicle speed conditions in the MDPS system. The horizontal axis refers to the steering angle indicating the position of the steering wheel. The vertical axis refers to the steering torque applied to the steering wheel.
Here, a (+) region of the steering angle may refer to a position of the steering wheel that is rotated to the right based on an on-center position (0°), and a (−) region of the steering angle may refer to a position of the steering wheel that is rotated to the left based on the on-center position.
In addition, (+) or (−) of the steering torque indicate the direction in which the driver applies steering torque to the steering wheel. As the steering angle of the steering wheel rotated to the right is increased, the steering torque in the (+) direction is gradually increased and then smoothly varied (refer to section S1).
As the steering angle of the steering wheel rotated to the left is increased, the steering torque in the (−) direction is gradually increased and then smoothly varied (in the graph, the absolute value of the torque is increased) (refer to section S2).
Of course, the fact that the steering torque applied from the driver to the steering wheel is increased means that, reversely, the auxiliary steering torque generated by the motor is reduced. The fact that the steering torque is reduced means that the auxiliary steering torque is increased.
In FIG. 1, ‘S1’ or ‘S2’ refers to a steering section in which the driver rotates the steering wheel from the on-center position in any one direction, and ‘R1’ or ‘R2’ refers to a restoration section in which the steering wheel that has been rotated by the manipulation of the driver is rotated to return to the on-center position.
In the typical MDPS system, to enhance the steering sensation that the driver feels, an additional process of compensating a motor control value determined based on sensor detection values or the like is performed. The operation of the motor is controlled to generate an auxiliary steering torque with a compensated control value as a target value.
In this regard, the motor control value may be the auxiliary steering torque that is obtained based on the sensor detection values or the like in a basic MDPS logic. In this case, after compensation torque is determined from information collected from the vehicle, the auxiliary steering torque obtained in the basic MDPS logic is compensated for using the compensation torque. The operation of the motor is controlled with the compensated torque as the target control value.
Here, the current of the motor is controlled such that the compensated target control value is outputted. The operation of the motor that outputs the auxiliary steering torque is controlled by applying the motor current corresponding to the compensated target control value to the motor.
Alternatively, the motor control value may be the motor current (assist control current) that is obtained based on the sensor detection values or the like in the basic MDPS logic. In this case, a current compensation rate is determined from information collected from the vehicle, and the motor current obtained in the basic MDPS logic is compensated for at the current compensation rate. Thereafter, the operation of the motor is controlled by applying the motor current thereto.
However, in the conventional MDPS system, in compensating for the motor control value to output the auxiliary steering torque, a method of compensating for the motor control value is not divided according to the distinction of the steering section and the restoration section.
That is, only a method of enhancing the steering sensation that the driver feels mainly for the steering section has been known, and a technology for separately controlling compensation operations for the steering section and the restoration section is not present.
Therefore, there is a problem in that although the driver can feel satisfactory steering sensation in the steering section, the steering sensation in the restoration section is unsatisfactory.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.